The present invention relates to a successive period-to-voltage converting apparatus which is suitable for use in the measurement of a variance in the speed of, for instance, a motor or engine.
The duration of an input pulse, i.e. its time length is usually measured by use of such an apparatus as disclosed in U.S. Pat. No. 4,611,926, for example. This apparatus includes a gate circuit, a clock generator, and a counter. With this apparatus, the gate circuit is kept open during the duration of the input pulse, the number of clock pulses from the clock generator, which have passed through the gate circuit, is counted by the counter, and then the count value is obtained as a value corresponding to the measured pulse width. The measurement accuracy of such an apparatus is dependent on the frequency of clock pulses used; for example, when a 10 MHz clock is used, resolution of the measured time length will be 100 nsec. At present, however, the frequency at which the counter is operable is 500 MHz at the highest, and when a 500 MHz clock is utilized, the accuracy of time measurement will be 2 nsec or so. Accordingly, it is difficult, with such a time measuring apparatus, to conduct the time measurement with a higher degree of accuracy.
Furthermore, the above-mentioned apparatus is so arranged as to successively measure the widths of a series of pulses. When this apparatus is applied to the measurement of pulse periods, for instance, only alternate pulse periods such as those from first to second pulse, from third to fourth pulse, from fifth to sixth pulse, etc. are usually measured, but the periods from the second to third pulse, from the fourth to fifth pulse, etc. are not measured.
This defect will constitute a serious obstacle to the measurement of a variance in the speed of a precision motor or accurate measurement of a variance in the speed of an engine.
For measuring a variance in the speed of a rotating machine, it is customary in the art to employ a method in which a pulse generator is provided in association with the rotary shaft to be measured, at least one pulse is generated by the pulse generator for each rotation of the rotary shaft, and the time intervals each between two successive such pulses are measured, thereby measuring the variance in the rotating speed.
With the afore-mentioned apparatus which can measure the time interval every other pulse alone, a variance in the speed of the rotary shaft is measured only every other rotation thereof. Accordingly, the state of rotation of the rotary shaft cannot continuously be measured.
Even if a plurality of pulses are produced for each rotation of the rotary shaft, it is still impossible to measure the state of a continuous variance in the speed of the rotary shaft during one rotation thereof.